i) Any discontinuity whatsoever can give stress concentrations and if in an area where stresses are already high will give an easy start for a crack.
ii) An oxyacetylene cut hole is probably the worst as the hole is irregular, which is already producing stress raisers, and by cooling down from the molten state and shrinking within a very stiff field means that there are probably a variety of small tension cracks within material already locally stressed to yield.
iii) Punched holes also provide a start point. As these are formed the stresses are well beyond yield (at ultimate). As the punch withdraws some of the perimeter of the hole remains at yield stress in tension and contains small cracks within this tension zone.
iv) Every weld produces a tiny groove along all of its edges, even with the best quality of weld and of course the extremities of the welds can be in areas of maximum thermal bending and maximum stress (shear and tension). This is not necessarily a question of quality of the weld. A perfect weld still has the tiny grooves and will still allow the initial liquid metal ingress.
v) Please note that these starting points are of no danger unless the stresses to pull the steel apart, and the "motor" to keep pulling them apart, coexist. There is no need to consider punched holes in cleats, or in thin end plates: or burned holes in stress free zones; or welds where there is no thermal stressing.